1. Field of the Invention
This invention relates generally to displays, and in particular to generating drive sequences for pulse width modulated displays
2. Background
The development of synthetic displays has been ongoing for many years. It is desired to faithfully and accurately reproduce the image of an object on the displays. One impediment to accurate reproduction of an image is non-linearity of intensity reproduction. A correction factor used in display systems to compensate for the non-linearity is referred to as gamma correction. In addition to non-linearity there are other artifacts in rendering an image on the display that can adversely affect the image. Two such artifacts are referred to as static image contours and dynamic false contouring (DFC).
Gamma can be defined as a numerical parameter that describes the non-linearity of intensity reproduction. Gamma correction can be applied to a display driving signal, or intensity value, to compensate for this non-linearity.
This definition of gamma is accurate for displays based on cathode ray tube (CRT) technology however in displays, such as liquid crystal displays, gamma correction has evolved to include all deviations from accurate reproduction of an image. For example, the Voltage-Transfer curve of a liquid crystal display is generally piecewise linear. Also, the digital to analog converters (DACs) that drive the displays are normally highly linear and the gamma function itself is exponential. In most instances this leads to the use of a lookup table since the relationship between data and luminance for such systems is not easily defined in closed form over the entire range of interest. The declining cost of memory has reduced this to the point of having little impact on the total cost of the display system.
Static image contours exist when an allocation of color codes to a display results in visible lines between sections of an image. The origins of this include having an insufficient number of gray levels available to represent an image and having an incorrect allocation of the color code to the final image. In an example of the former, representing 8 bit color on a display with 5 bits of color depth creates jumps in the appearance of an image where the image should appear to be smoothly shaded. In the latter the appearance is better but there are still these contour lines present in some instances.
Dynamic false contouring (DFC) is an artifact in pulse width modulated displays that is caused by human perception of adjacent pixels in motion where the data written on adjacent pixels are displaced in time substantially. DFC causes pixels to be perceived at incorrect brightness levels when adjacent pixels are presented temporally out of phase to each other. Displays, such as digital liquid crystal displays, that use pulse width modulation to vary the intensity, or gray level, of pixels of the displays are prone to DFC.
A variety of digital liquid crystal display systems have developed over time. Digital liquid crystal displays encompass displays that use pulse width modulation to develop gray scale. These devices typically select between two voltages to switch between a bright state and a dark state, with the gray scale being determined by the time ratio of the two states. The majority of displays that use this type of modulation use either ferroelectric liquid crystals (FLC) in a surface stabilized FLC (SSFLC) configuration or nematic liquid crystals in a variety of other configurations.
Generation of gray scale using pulse width modulation with liquid crystal display devices is quite similar to the techniques used to generate pulse width modulation for plasma panels and other PWM devices such as the Texas Instrument digital light projector (DLP) micro-mirror device.
Plasma panel displays operate a simpler form of pulse width modulation. The plasma pixel cells operate in an on/off constant intensity mode. Each pixel is preloaded with a charge that determines whether or not it will ring up. The high voltage is then released to the cells and those that are pre-charged ring up to their uniform luminance state. At the end of display period another circuit extinguishes the plasma state and the next pixel pre-charge cycle begins. This means that there is always a “dead time” after a display state to permit pre-charge of the next pixels to be displayed in the next display state. This type of display is also prone to DFC.
As noted, DFC is caused by the human perception of adjacent pixels in motion where the data on the two pixels are displaced in time substantially. There are numerous techniques proposed to counter this defect. Many techniques involve temporally splitting higher order bits, for example, displaying a drive value of 128 out of a possible maximum drive value of 255 (128/255) in two segments, each a nominal drive value of 64 (64/255), with some temporal separation between the two segments. Other techniques include adding additional non-binary weighted planes to provide redundant data paths for the creation of individual gray scales. These techniques all have an adverse consequence of reducing the brightness of the system because each additional plane of data requires an additional pre-charge cycle during which no data can be shown. As a consequence these techniques often require that one or more lower order bits be dropped, thus reducing the gray scale depth of the system.
Digital micro-mirror devices, such as DLP, suffer from defects similar to those seen in plasma panels. The available techniques to counter the defects are different because a backplane on micro-mirror devices can be loaded with new data while current data is being shown. A fairly effective solution is to raise the frame rate, which has been shown empirically to reduce the visibility of dynamics false contouring. Most prominent defect seen in DLP devices are motion artifacts seen in field sequential color, such as color breakup.
Liquid crystal (LC) display devices operate in a manner analogous to the above devices. The digital data for the display is parsed in the form of pulse width modulation to form a time varying LC state that modulates the polarization state of light incumbent on it and thus creates gray scale. SSFLC devices are similar to plasma panels and micro-mirror devices because the electrical modulation waveform and the light modulation are quite similar. SSFLC devices typically have rapid, symmetrical on/off times that make static gray scale images quite uniform, although dynamic images still suffer from the dynamic false contouring artifact. Because of the unique spontaneous polarization characteristic of the SSFLC under drive, DC balancing of the display causes it to display the inverse of the image during such times. Thus, techniques are employed to insure the inverse image is not displayed, which is one reason why SSFLC projection display devices have not become widespread in display systems where the lamp intensity cannot be easily modulated. Uniquely, the rise and fall times of SSFLC cells are identical, largely because, as is well known in the art, the common plane ITO voltage is intermediate between the dark state voltage and the bright state voltage.
Backplane devices capable of delivering pulse width modulation are well known in the art. Examples are disclosed in US Patent Application Publication No. 2004/0125090 and 2004/0125094, both assigned to the Assignee of the present application and incorporated by reference herein, in their entirety. Pulse width modulation sequences that can be applied to such backplanes are disclosed in U.S. patent application Ser. No. 2003/0210257 which is incorporated by reference herein, in its entirety.) These devices are quite similar to digital memory devices and they may use either DRAM or SRAM technology to store a value in the pixel circuitry that selects either a dark state voltage or a bright state voltage.
Nematic liquid crystal devices, when driven with pulse width modulation, are similar in some respects to the previous mentioned displays. A fundamental difference is that the electrical waveform used to drive the liquid crystal operates at a frequency much higher than the frequency response of the liquid crystal itself The net effect is that the liquid crystal modulation effects are a smoothed version of the envelope of the drive waveform. The degree of smoothing depends in large measure on the step response of the liquid crystal device. Typical device speed may very from as fast as 250 microseconds to as slow as 30 milliseconds, so the range of variation is enormous.
Nematic liquid crystal devices have substantial rise and fall times, which can affect the appearance of the display. Also, nematic microdisplays can be prone to lateral field artifacts because the ratio of the cell gap to the pixel pitch is generally significantly higher than in direct view devices. Direct view devices suffer similar artifacts but they form a small part of the image. Nematic devices also are still liable to exhibit dynamic false contouring in those instances where the nematic liquid crystal response time is sufficiently fast. Finally, the material has an inherent deviation from the ideal of the gamma curve. While not a defect per se it is a circumstance that requires adaptation.
Therefore, there is a need for improved systems, apparatus, and techniques for improved gray scale drive sequence for pulse width modulated displays.